As a light emitting element using an electroluminescence phenomenon, an electroluminescence (hereinafter, referred to as “EL”) element is known. An EL element is capable of emitting light of a color of any of various wavelengths in accordance with a light emitting material selected to form a light emitting layer, and is now applied more and more for a display device, an illumination device and the like. Especially, an organic EL element using an organic material as the light emitting material is now a target of attention.
A display device using an organic EL element includes the organic EL element acting as a light emitting element and a switching element that controls light emission performed by the EL element, in each of pixels located in a matrix on a substrate. The switching element is controlled on a pixel-by-pixel basis, so that an arbitrary image is displayed on the entirety of a display area.
As a display form of a display device, two types of form, namely, a top emission type and a bottom emission type, are known. With a top emission type display device, light that is emitted by an organic EL element is reflected by a pixel electrode and output outside. With a bottom emission type display device, light that is emitted by an organic EL element is transmitted through the pixel electrode and output outside. Especially, the top emission type display device is advantageous in having a high numerical aperture of the pixels.
With the top emission type display device, light needs to be transmitted through a common electrode (cathode) that forms a pair with a pixel electrode (anode) before being output outside. Therefore, the top emission type display device often uses a thin film formed of MgAg (magnesium-silver alloy) or a transparent conductive film formed of ITO (indium tin oxide), IZO (indium zinc oxide) or the like as a common electrode.
Among these films, the transparent conductive film formed of ITO, IZO or the like has a higher resistance that that of a metal film. Therefore, in the case where the level of current consumed by the display area is high because of a large size of the screen, a significant voltage drop is caused in, or in the vicinity of a central area, namely, an area far from the current supply source. This influences the display.
Japanese Laid-Open Patent Publication No. 2009-276721 discloses a technology of adjusting the resistivity of a transparent conductive film formed of ITO, IZO or the like in order to solve such a problem. This technology is to provide a low-resistance material on a cathode to form an assisting line layer.